1. Field of the Invention
An exemplary embodiment of the invention relates to a liquid crystal display and a method of driving the same.
2. Discussion of the Related Art
Active matrix type liquid crystal displays display a moving picture using a thin film transistor (TFT) as a switching element. The active matrix type liquid crystal displays have been implemented in televisions as well as display devices in portable devices, such as office equipment and computers, because of the thin profile of the active matrix type liquid crystal displays. Accordingly, cathode ray tubes (CRT) are being replaced by active matrix type liquid crystal displays.
The active matrix type liquid crystal display includes data lines and gate lines crossing each other, and liquid crystal cells arranged at each crossing of the data lines and the gate lines in a matrix format. A thin film transistor (TFT) is formed at each crossing of the data lines and the gate lines. As shown in FIG. 1, data driver integrated circuits (ICs) of a liquid crystal display alternately supply a positive data voltage and a negative data voltage to the data lines during a low logic period of a source output enable signal SOE. Gate driver ICs of the liquid crystal display sequentially supply gate pulses synchronized with the positive and negative data voltages to the gate lines during a low logic period of a gate output enable signal GOE. The gate pulses are sequentially supplied to first to n-th gate lines to scan from an uppermost line of the display screen to a lowermost line of the display screen.
If a DC voltage is applied to a liquid crystal layer of the liquid crystal display for a long time, negative ions move in the same vector direction and positive ions move in the same vector direction opposite the vector direction of the negative ions depending on a polarity of an electric field applied to liquid crystals. Hence, the ions inside the liquid crystal layer are polarized. As time elapses, the accumulation of negative ions and the accumulation of positive ions increase. As a result, an alignment layer is degraded and alignment characteristics of the liquid crystal are degraded. In other words, if the DC voltage is applied to the liquid crystal display for a long time, stains appear on a display image, and the stains spread as time elapses.
FIG. 2A shows a mosaic pattern of test data for generating stains in a stain test process. In the mosaic pattern, black gray level blocks with the uniform size and white gray level blocks with the uniform size are alternately positioned upward and downward and right and left. If the liquid crystal display displays the mosaic pattern for a long tine, stains appear in interfaces between the black gray level blocks and the white gray level blocks. Further, as time elapses, the stains spread in a transverse direction. In particular, the stains spreading in the transverse direction, as shown in FIGS. 2A and 2B, appear in the interfaces between the black gray level blocks and the white gray level blocks when the scanning of the data voltages moves from the black gray level blocks to the white gray level blocks, but do not appear when the scanning of the data voltages moves from the white gray level blocks to the black gray level blocks. In FIG. 2B, a line number indicates each line number of the liquid crystal display, namely, a row number of liquid crystal cells, and a numeral inscribed on the black gray level blocks and the white gray level blocks indicates the scan order of the data voltages. The liquid crystal cell adjacent to the liquid crystal cell of the black gray level block is charged to a white gray level voltage while the liquid crystal cell of the black gray level block is hold at a black gray level voltage, and ionic impurities mixed with the adjacent liquid crystal cells are polarized. The polarization adversely affects the alignment layer, and thus the stains appear.
The development of a liquid crystal material with a low dielectric constant or a method for improving an alignment material or an alignment method has been attempted so as to remove the stains. However, the method requires much time and expense to develop the maternal, and a reduction in a dielectric constant of the liquid crystal may cause a reduction in driving characteristics of the liquid crystal. According to the experimental findings, the appearance of the stains due to the polarization and the accumulation of the ions becomes rapider as the amount of impurities ionized inside the liquid crystal layer increases and a value of an acceleration fact increase. The acceleration fact includes a temperature, time, a DC drive of the liquid crystal, and the like. Accordingly, the stains rapidly appear and a stain level increases as a temperature rises or time for which a DC voltage with the same polarity is applied to the liquid crystal layer increases. Furthermore, because levels and shapes of the stains in the same model panels produced through the same production line are different from each other, the stain cannot be solved by the development of new material or an improvement in the manufacturing method.